Page 173 - Introduction to Marine Engineering

Fuel oils, lubricating oils and their treatment 159
Various designs utilise an impact or rolling action to break down the
fuel particles and mix them with the water. It is also considered that
agglomerates of asphaltenes and bituminous matter are broken down
and can therefore be burnt. The manufacturers contend that a
homogeniser can render a sludge burnable whereas a centrifuge would
remove such material. Homogenisers are able to reduce catalytic fines
into finely ground particles which will do no harm.
Shipboard experience with homogenisers is limited and generally not
favourable. Most authorities consider it better to remove water and solid
contaminants than simply grind them down.

Blenders

Blending is the mixing of two fuels, usually a heavy fuel and marine
diesel oil. The intention is to produce an intermediate-viscosity fuel
suitable for use in auxiliary diesels. The fuel cost savings for
intermediate fuel grades are sufficient to justify the cost of the blending
plant. Furthermore no supply problems exist since the appropriate
mixture can be produced by the blender from available heavy and
marine diesel oils.
The blending unit thoroughly mixes the two fuels in the appropriate
proportions before supplying it to a blended fuel supply tank.
Compatibility can be a problem and tests should be conducted on any
new fuel before it is used. Incompatible fuels may produce sludge or
sediment. The cracked residues presently supplied from many
refineries are very prone to incompatibility problems when blended with
marine diesel oil.

Filters and strainers

Mechanical separation of solid contaminants from oil systems (fuel and
lubricating) is achieved by the use of filters and strainers. A strainer is
usually a coarse filter to remove the larger contaminating particles. Both
are arranged as full flow units, usually mounted in pairs (duplex) with
one as a standby.
The strainer usually employs a mesh screen, an assembly of closely
packed metal plates or wire coils which effectively block all but the
smallest particles. It is usually fitted on the suction side of a pump and
must be cleaned regularly or when the pressure differential across it
become unacceptable. Where suction conditions are critical the strainer
will be fitted on the discharge side of the pump. When cleaning is
undertaken the other unit will be connected into the system by